1. Brief Description of the Prior Art
In the search for petroleum, large amounts of natural gas are discovered in remote areas where there is no local market for it. The dominant technology now employed for utilizing remote natural gas involves its conversion to synthesis gas, a mixture of hydrogen and carbon monoxide. While syngas-based processes fulfill the need for an easily transportable liquid that can be converted to several useful products, synthesis gas is an expensive intermediate. Oxygen can be add.RTM.d with advantage to the rather inert methane molecule when products such as methanol or acetic acid are desired. In the case of hydrocarbons such as gasoline or diesel fuel, however, processes based on synthesis gas essentially require the addition of oxygen, followed by its removal, increasing final product cost.
Methane, the predominant component of natural gas, although difficult to activate, can be reacted with oxygen or oxygen-containing compounds such as water or carbon dioxide to produce synthesis gas in a process known generally as reforming. This mixture can be converted to higher hydrocarbons using, for example, Fischer-Tropsch technology, and then upgraded to transportation fuels using usual refining methods. Alternatively, the mixture can be converted to liquid oxygenates which in turn can be converted to more conventional transportation fuels by catalysts such as certain zeolites.
Because reforming requires high capital investment and energy inefficient processing (as in steam reforming, where fuel is burned to supply heat of reforming) and represents an indirect route to the production of hydrocarbons, other means of converting methane directly to higher hydrocarbons are needed.
Oxidative coupling has been recognized as a promising approach to the problem of methane conversion although its mechanism is not completely understood. In such processes, methane is contacted with solid materials referred to by various terms including catalysts, promoted-catalysts, activators, conversion catalysts, or upgrading catalysts. Methane mixed with oxygen and allowed to contact the catalyst is directly converted to ethane, ethylene, higher hydrocarbons and water. The conversion of methane to carbon dioxide, which is, in essence, the highly favored thermodynamically process of combustion, is undesirable as both oxygen and carbon are consumed without producing the desired higher value C.sub.2+ hydrocarbons. In order to avoid complete combustion, many methods for oxidative conversion have been carried out in the absence of an oxygen containing gas, relying on the oxygen supplied by an oxide catalyst itself. Such catalysts can then be regenerated (off cycle) by re-oxidation.
Catalytic mixtures of yttrium-barium-copper oxides are highly active and 100% selective for producing CO.sub.2 that is, they are combustion catalysts. In order to obtain the required selectivity to hydrocarbon formation, Group IA metals, particularly lithium and sodium, have been used in such catalytic mixtures. Under the conditions used for oxidative coupling, however, migration and loss of the alkali metal normally occurs. Thus there is a need for highly active, C.sub.2+ hydrocarbon-selective and stable oxidative coupling catalysts, and new or improved processes for these.
A three-component catalyst for the oxidative conversion of methane to hydrocarbons containing 2 or more carbon atoms is disclosed in U.S. Pat. Nos. 5,024,984 and 5,059,740 by Kaminsky et al. Improvements in methanation reaction are disclosed in U.S. Pat. No. 4,331,544 to Takaya et al. A process for the removal of sulfur oxides from a gas is disclosed in U.S. Pat. No. 4,957,718 to Yoo et al. A variety of patents covering methane oxidative coupling, such as U.S. Pat. Nos. 4,956,327 and 4,826,796 to Erekson et al, U.S. Pat. No. 4,971,940 to Kaminsky et al and U.S. Pat. Nos. 5,068,215 and 4,886,931 to Bartek et al. Catalyst conversion is also discussed in "The Conversion of Methane to Ethylene and Ethane with Near Total Selectivity by Low Temperature (&lt;610.degree. C.) Oxydehydrogenation over a Calcium-Nickel-Potassium Oxide Catalyst," J.C. Baltzer A.G. Scientific Publishing Company, page 225-262, P. Pereira, S.H. Lee, G. A. Somorjai and Heinz Heinemann, Jul. 1990, and New Cost-Effective Methane Conversion Process, New Technology Announcement, Lawrence Berkeley Laboratory. A process in which no carbon oxides are produced at all, however offer some disadvantages in the context of processes located at remote well sites. In particular, heat would need to be generated outside of the process unit and transferred to the reactor. This is generally an inefficient process.
A fluidized bed offers a unique chemical environment for gas-solid reactions by providing efficient contact between the gas and the solid phases while offering excellent rates of heat-dissipation. For these reasons the fluidized-bed reactor is the reactor configuration of choice for many exothermic reactions, including catalytic oxidations such as the partial oxidation of naphthalene to produce phthalic anhydride, and noncatalytic oxidation reactions such as coal combustion. Despite this history, however, very little work has been reported on the use of a fluidized bed for methane oxidative coupling (MOC).
Much of the research on MOC has focused on the identification of more active and/or more selective catalyst formulations in an effort to improve the overall yield of higher hydrocarbons. Virtually all of this work has involved the use of fixed-bed microreactors for testing while the metal oxides under investigation have been quite exotic or expensive. Clearly, a considerable materials-development effort would be required to prepare an attrition-resistant catalyst from any of these formulations, which would be suitable for fluidized bed applications.
MOC has received considerable attention in recent years initiated, in large part, by the work of G. E. Keller and M. M. Bhasin, Synthesis of Ethylene via Oxidative Coupling of Methane, Journal of Catalysis 73, 9-19 (1982) and John A. Sofranko, John J. Leonard and C. Andrew Jones, The Oxidative Conversion of Methane to Higher Hydrocarbons, Journal of Catalysis 103, 302-310, (1987). These works focused on screening many different metal oxide catalysts on the basis of ethane/ethylene (C.sub.2,total) yield according to the reaction, EQU nCH.sub.4 +O.sub.2 ==.fwdarw.C.sub.n H.sub.4n-y +yH.sub.2 O(1)
Work since that time has generally centered on metals identified in these studies although more exotic metals or mixtures of these have been considered. Most work has been in fixed-bed microreactors despite the fact that a fluidized-bed reactor would likely be the preferred commercial reactor.